Selective Grain Boundary Engineering

ABSTRACT

A process for grain boundary engineering of an aluminum alloy of AA5XXX series which includes steps of annealing the aluminum alloy at a first temperature of from about 350° C. to about 450° C.; deforming the annealed aluminum alloy to reduce the thickness by from about 2% to about 20% of the original thickness of the aluminum alloy; heat treating the deformed aluminum alloy at a second temperature from about 450° C. to about 550° C., and optionally sensitizing the heat treated alloy in one or more sensitizing steps. Aluminum alloys of the AA5XXX series treated by the process of the present invention are also provided.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.14/910,272, filed on Feb. 5, 2016, which, in turn, is a 371 continuationof International Application No. PCT/US14/52033, filed Aug. 21, 2014,which claims the benefit of U.S. Provisional Application No. 61/868,212,filed Aug. 21, 2013, the entire disclosures of which are herebyincorporated by reference as if set forth fully herein.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with government support under Contract No.N000141210505 awarded by the Office of Naval Research. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention is directed to the field of thermomechanicalprocessing of aluminum alloy. In particular, the present invention isdirected to a thermomechanical process for treating aluminum alloy todecrease the susceptibility of the aluminum alloy to intergranularcorrosion.

2. Description of the Related Technology

Due to their light weight and good corrosion resistance properties ascompared to steels, aluminum alloys have the potential of replacingsteels in a wide range of applications, such as in aircraft, ships andboats, trucks, cars and other vehicles. For example, aluminum alloys ofthe AA5XXX series, which have magnesium as a major alloying element,have been broadly used in marine applications. AA5XXX series aluminumalloys may be sensitized to enhance resistance to corrosion, as a resultof which magnesium atoms in Al—Mg alloys may precipitate out and form aβ-phase (Al₃Mg₂) along the grain boundaries, especially in alloys withMg levels above ˜3 wt. %.

The β-phase is anodic to the matrix material, leading to formation ofsmall openings along the grain boundary network. These small openingswithin the matrix material may initiate a process called stresscorrosion cracking (SCC). SCC is a type of cracking induced by thecombined influence of tensile stress and a corrosive environment. SCCcan lead to unexpected sudden failure of normally ductile metals whenthey are subjected to a tensile stress, due to the ability of SCC tocause gaps to grow faster within the aluminum alloy than otherwiseexpected.

To increase strength and/or corrosion resistance, aluminum alloys aretraditionally cast into an ingot of approximately 12 to 28 inches inthickness. The ingot is then scalped and preheated, after which it maybe hot rolled to about 0.125 inch, cold rolled to about 0.020 to 0.060inch, and subjected to further heat treatment, such as batch annealingor solution heat treatment. One concern with such traditional approachesis the intermetallic particles present in the as-cast aluminum alloyingots, which is a function of the alloy composition and thesolidification rate in the casting process. The intermetallic particlescan participate in a fracture initiation and propagation and, as aresult, may limit the formability or design tolerance of the aluminumalloys. The intermetallic particles also act as void nucleation sitesduring sheet forming processes, such as stretching and, therefore, maycontribute to fracture initiation within the aluminum alloy matrix.

Several thermomechanical processes have been developed attempting toimprove various properties of aluminum alloys, especially theircorrosion resistance. For example, U.S. Pat. No. 6,544,358 disclosesseveral methods of processing aluminum alloy of the AA5XXX One methoduses two steps of hot rolling and one step of cold rolling to produce athin alloy sheet. The alloy sheet is then inter-annealed at atemperature of from 300 to 500° C. After annealing, the alloy sheet iscold rolled to reduce the thickness by 40-60%; and lastly, the coldrolled alloy is annealed at temperature from 300 to 500° C.

L. Tan & T. R. Allen, “Effect of thermomechanical treatment on thecorrosion of AA5083,” Corrosion Science, vol. 52, pages 548-554 (2010)discloses a process of treating aluminum alloy AA5083-H116 to induceresistance to corrosion. The process includes the steps of cold workingto reduce the thickness by about 25%, followed by annealing at 500° C.for 30 minutes and water quenching. The process is said to affect grainboundary characteristics, grain shape, texture, and precipitates formedby corrosion of the aluminum alloys.

U.S. Pat. No. 6,344,096 discloses a method for treating aluminum alloysincluding alloys of the AA5XXX series. The method involves thesequential steps of cold rolling the alloy to provide a sheet with athickness of less than 0.15 inch starting from an alloy with thicknessof 0.5 inch and continuous annealing at about 700 to 1100° F. for 1 to60 seconds in order to cause one or more of the alloying constituents toenter solid solution. This is followed by sufficiently rapid cooling tominimize undesired constituent precipitation. Such cooling may beeffected, for example, by forced air, water spray or water mist. Thetreated aluminum alloy sheet is said to have high strength andformability along with good surface quality.

U.S. Pat. No. 5,496,426 discloses a process said to improve thestrength, toughness and corrosion resistance of an aluminum alloy. Theprocess includes the successive steps of homogenizing, hot rolling andthermally treating or annealing at about 750° to 850° F. for 1 to 6hours; followed by cold rolling to a reduction in thickness of betweenabout 20 and 60%; followed by a two-stage thermal annealing treatmentincluding heating, preferably within a temperature range of about 625 toabout 725° F. for a period of about 1½ to 4 hours, followed bycontrolled cooling or ramping down to one or more temperatures within arange of 350 to 550° F. over a period of about 2 to 6 hours.

U.S. Pat. No. 6,350,329 discloses a process for treating age-hardenedaluminum alloys, such as those of the AA2XXX, AA6XXX, AA7XXX and certainAA8XXX series. The process includes the successive steps of solutionheat treating the alloy at about 540° C. for about one hour, rapidlycooling the alloy, plastically deforming the alloy by rolling at roomtemperature, under conditions sufficiently severe to produce ahigh-energy defect structure in the grains of the alloy, and aging thealloy to induce nucleation and growth of precipitates at a temperatureof 380 to 450° C. for up to 24 hours. The process is preferably coupledwith a multi-step low and high temperature aging process, and isdesigned to produce a uniform distribution of micron-size precipitatesnecessary for the subsequent development of a fine, equiaxed grainstructure that is said to be stable at superplastic formingtemperatures.

WO 2009/132436 discloses a thermomechanical process for treating anAA6XXX aluminum alloy to produce extended high temperature ductility.The process includes the sequential steps of solution heat treating thealloy followed by rapid cooling, plastically deforming the alloy by atleast 30%, annealing by heating from a room temperature to high enoughtemperature (e.g. 300° C.) to induce re-crystallization and/or formationof precipitates, continuously heating the alloy at below there-crystallization temperature for forming dispersed fine precipitates,and heating the alloy at or above the crystallization temperature toachieve a fine grain structure.

To improve corrosion resistance of the aluminum alloy in the AA5XXXseries, the present invention employs a thermomechanical process totreat the aluminum alloy to selectively engineer the grain boundary. Theengineered grain boundary impedes growth of β-phase in the alloy matrix.This process is capable of significantly reducing the aluminum alloy'ssusceptibility to sensitization, thus increasing its resistance tocorrosion.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a process for grainboundary engineering of an aluminum alloy of the AA5XXX series. Theprocess includes steps of annealing the aluminum alloy at a temperatureof from about 350° C. to about 450° C.; deforming the annealed aluminumalloy to reduce the thickness by from about 2% to about 20% of theoriginal thickness of the aluminum alloy; and heat treating the deformedaluminum alloy at a temperature of from about 450° C. to about 550° C.

In another aspect, the process of the present invention further includesthe step of cooling the annealed aluminum alloy prior to the deformingstep.

In yet another aspect, the process of the present invention furtherincludes the step of sensitizing the heat treated aluminum alloy.

In yet another aspect, the process of the present invention includes aplurality of sensitizing steps that are each performed at a differenttemperature.

In yet another aspect, the present invention provides an aluminum alloyof the AA5XXX series treated by the process of the present invention.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart representing a process for selective grainboundary engineering of an aluminum alloy of the AA5XXX series accordingto the present invention.

FIG. 2 is a plot of the mass loss analysis of samples processed inaccordance with the method of Example 1.

FIG. 3 is a plot of the mass loss analysis for samples processed as inExamples 1 and 2 using a cold rolling thickness reduction in the rangeof 0%-20%.

FIG. 4A shows an electron backscatter diffraction image obtained byusing a scanning electron microscope on an unprocessed aluminum alloysample.

FIG. 4B shows an electron backscatter diffraction image obtained byusing a scanning electron microscope on an aluminum alloy sampleprocessed with a 5% cold rolling thickness reduction and 500° C. heattreatment as set forth in Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present disclosure aredescribed by referencing various exemplary embodiments. Although certainembodiments are specifically described herein, one of ordinary skill inthe art will readily recognize that the same principles are equallyapplicable to, and can be employed in other systems and methods. Beforeexplaining the disclosed embodiments of the present disclosure indetail, it is to be understood that the disclosure is not limited in itsapplication to the details of any particular embodiment shown.Additionally, the terminology used herein is for the purpose ofdescription and not of limitation. Furthermore, although certain methodsare described with reference to steps that are presented herein in acertain order, in many instances, these steps may be performed in anyorder as may be appreciated by one skilled in the art; the novel methodis therefore not limited to the particular arrangement of stepsdisclosed herein.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural references unless thecontext clearly dictates otherwise. Furthermore, the terms “a” (or“an”), “one or more” and “at least one” can be used interchangeablyherein. The terms “comprising”, “including”, “having” and “constructedfrom” can also be used interchangeably.

The present invention provides a process for selective grain boundaryengineering of aluminum alloys for enhancing corrosion resistance.Aluminum alloys suitable for the process of the present inventioninclude aluminum alloys of the AA5XXX series in the Aluminum AssociationRegister. The AA5XXX series aluminum alloys have magnesium as the majoralloying element.

The AA5XXX series of alloys are considered non-heat treatable becausethey cannot generally be appreciably strengthened by solution heattreatment. Instead, the AA5XXX series aluminum alloys are usuallystrengthened by solid-solution formation of second-phase microstructuralconstituents, dispersoid precipitates and/or strain hardening. Inaddition the AA5XXX series is readily weldable, and for these reasonsthese alloys may be used in a wide variety of applications such asshipbuilding, transportation, pressure vessels, bridges and buildings.These alloys are often welded with filler alloys, which are selectedafter consideration of the magnesium content of the base material, andthe application and service conditions of the welded aluminum alloy.

Examples of suitable aluminum alloy include the aluminum alloy AA5083,for example, AA5083-H116. Aluminum alloy AA5083 is a commerciallyavailable aluminum alloy comprising about 4.4% Mg, 0.7% Mn, 0.15% Cr,with the balance being aluminum. This alloy is currently used in marineapplications because it is considered to be resistant to seawatercorrosion. The temper designation “H116” indicates a soft temper alloythat has been strain hardened without thermal treatment.

In one aspect, the present invention provides a process for selectivegrain boundary engineering of an aluminum alloy of the AA5XXX series.Referring to FIG. 1, the process comprises steps of annealing thealuminum alloy at a temperature of from about 350° C. to about 450° C.;deforming the annealed aluminum alloy to reduce the thickness of thealuminum alloy by from about 2% to about 20% of the original thicknessof the aluminum alloy; and heat treating the deformed aluminum alloy ata temperature of from about 450° C. to about 550° C.

In some embodiments, the initial annealing step comprises solution heattreating the aluminum alloy at a temperature of from about 350° C. toabout 450° C., or from about 370° C. to about 430° C., or from about380° C. to about 420° C., or from about 390° C. to about 410° C. Thetime required for the annealing step should be sufficient to ensure thatall magnesium in the aluminum alloy has gone into solution and that nopreviously formed β-phase from, for example, fabrication or shaping ispresent in the annealed aluminum alloy. A skilled person may ascertainthe annealing time for a specific aluminum alloy. To assure that themagnesium is in solution one can maintain the temperature in the higherend of the temperature ranges given above, i.e. above about 400° C.Hardness testing can give some indication that the precipitates are goneas a result of the magnesium going back into solution. G67 corrosiontesting can also be used to show that magnesium precipitates are gone asa result of the magnesium going back into solution.

In some embodiments, the annealing step is conducted for a period offrom about 0.5 hour to about 3 hours, or from about 1 to about 2 hours.

In some embodiments, the annealing step may be performed in a solutionheat treating furnace. The solution heat treating furnace must becapable of accurately controlling the furnace temperature andtemperature variation must be limited to within a range of about ±5° C.Overheating should be avoided, especially not exceeding the initialeutectic melting temperatures of the aluminum alloy. Though not apparentat early stages of overheating, a deterioration of mechanical propertiesof the aluminum alloy may result from overheating the aluminum alloy.

After the annealing step, the aluminum alloy is cooled by, for example,air cooling or quenching. Although the mode of cooling is not critical,the annealed aluminum alloy is preferably cooled rapidly to atemperature at which the diffusion rate of the elements in the aluminumalloy matrix is not appreciable, and formation of precipitates,particularly on the grain boundaries of the aluminum alloy, is therebyprevented. In some embodiments, the cooling rate may be from about 10 toabout 30° C. per minute, or from about 15 to about 25° C. per minute, orfrom about 18 to about 22° C. per minute, until the temperature of thealuminum alloy is reduced to below a desired temperature. This desiredtemperature may be from about 200° C. to about 300° C., or from about230° C. to about 290° C., or from about 250° C. to about 290° C., insome embodiments. It is desirable to ensure that magnesium precipitationdoes not occur during cooling and thus ensuring a temperature below 200°C. Temperatures of 100° C. to 200° C. can usually be tolerated for aperiod of up to about 10 minutes while minimizing or prevent magnesiumprecipitation.

In one embodiment, the annealed aluminum alloy is quenched. Quenchingcan help to keep dissolved constituents in solution after cooling theannealed alloy to room temperature. The speed of quenching is importantas excessive delay in transferring the aluminum alloys to a quenchingmedium may adversely affect the properties of the aluminum alloy. It isdesirable to reduce the temperature to below 200° C. as soon as possiblein order to minimize or prevent magnesium precipitation and thusquenching may be employed to achieve this goal. The quenching medium maybe, for example, water or oil at room temperature. Other rapid coolingmethods known to skilled persons may also be used for the presentinvention.

Referring to FIG. 1, the annealed aluminum alloy is then deformed toachieve a reduction in thickness of from about 2% to about 20%, or fromabout 3 to about 15%, or from about 5% to about 10% of the originalthickness of the aluminum alloy. Any suitable method for deforming analloy may be used, such as rolling, stretching, extrusion, pressing,drawing, forging, torsion processes, and any combination of theseprocesses, among others, so long as the method is sufficiently severe toproduce desired thickness reduction in the aluminum alloy. Preferably,the amount of reduction per pass and number of passes is such that thedeformation fully penetrates, or substantially penetrates, the entirethickness of the aluminum alloy. It is also preferable that thedeformation be uniform, or substantially uniform, throughout thethickness of the aluminum alloy.

In one exemplary embodiment, the deforming step comprises a cold rollingprocess, including both conventional cold rolling and asymmetric coldrolling. Cold rolling is a metal working process in which a metal isdeformed by passing it through rollers at a temperature below itsrecrystallization temperature, such as at room temperature. Cold rollingincreases the yield strength and hardness of the aluminum alloy byintroducing defects into the alloy's crystal structure. These defectsprevent further slip and can reduce the grain size of the aluminumalloy, resulting in improved hardness of the aluminum alloy. The coldrolling can be carried out in one or more roll bite passes. Any mode ofcold rolling may be used, so long as it is sufficiently severe toproduce the required thickness reduction. The advantages produced bycold rolling are dimensional accuracy and good surface finish.

Referring to FIG. 1, the deformed aluminum alloy is then heat treated ata temperature of from about 450° C. to about 550° C., or from about 475°C. to about 525° C., or from about 485° C. to about 515° C., or about500° C. The time for this heat treatment step may be from about 1 toabout 6 hours, or from about 2 to about 5 hours, or about 2 to about 4heat treatment step may be carried out in a batch operation foreconomical efficiency. The cooling of the aluminum alloy after this heattreating step is not critical and the alloy may be slowly cooled to roomtemperature.

The heat treated alluminum alloy may optionally be subjected to asensitizing step where the aluminum alloy is kept at a temperature offrom about 100° C. to about 200° C., or about 120° C. to about 180° C.,or about 140° C. to about 160° C. The sensitizing step may be performedfor a period of from about 70 hours to about 150 hours, or from about 80hours to about 130 hours, or from about 90 hours to about 120 hours. Thetimes and temperatures of the sensitizing step may vary depending on theparticular alloy. Also, lower sensitizing temperatures generally requirelonger sensitizing time periods, whereas higher sensitizing temperaturesgenerally require shorter sensitizing time periods to achieve comparableeffects.

In come embodiments, the sensitizing step can be carried out as aplurality of sensitizing steps, such that a sensitizing step at arelatively low temperature may be used to form a fine distribution ofgrains in the aluminum alloy, followed by one or more subsequentsensitizing steps at successively higher temperatures which may be usedto increase the speed of coarsening once precipitates have been formedin order to provide sufficiently coarse grains. In these embodiments,beginning the sensitizing step with a relatively lower temperatureincreases the driving force for grain formation, thereby increasing thenumber density of grains, and continuing the sensitizing process with arelatively higher temperature decreases the sensitizing time and mayalso enhance the economy of the process.

The alluminum alloy may be cooled after each sensitizing step, typicallyby air cooling, which is also easier and less-costly to implement thanother cooling methods.

The aluminum alloys treated by the process of the present invention havesuperior corrosion resistance. The corrosion resistance of the treatedaluminum alloy may be assessed by a known mass loss test in compliancewith the ASTM G67 standard.

To prepare the alloys for the mass loss test, a desmutting step may becarried out to remove residual substances on the sample's surface. Asample may be desmutted by placing the alloy in a sodium hydroxidesolution and then in nitric acid for sufficient time.

The grain structure of the treated aluminum alloys may be analyzed byusing a scanning electron microscope for electron backscatterdiffraction (EBSD). To achieve electron diffraction standard polishing,mount procedures may be used with a final 0.03 micron colloidal silicapolishing.

The present invention produces aluminum alloys that have a substantiallyreduced susceptibility to corrosion, especially seawater corrosion, insome cases by at least a factor of 2. The aluminum alloy treated by theprocess of the present invention is particularly suitable for navalvessels where seawater corrosion is of a significant concern. Thus, useof this process on an industrial scale to produce highly corrosionresistant aluminum alloy for naval vessels could reduce maintenance onnaval vessels due to seawater corrosion.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

EXAMPLES

The following examples are illustrative, but not limiting, of themethods and compositions of the present disclosure. Other suitablemodifications and adaptations of the variety of conditions andparameters normally encountered in the field are within the scope of thedisclosure.

Example 1

Aluminum alloy AA5083-H116 was treated using the process according tothe present invention. A control sample was not treated, but cut to sizeand sensitized for comparison. Another control sample was cut to sizebut not sensitized or treated.

Each sample was cut and ground to a thickness larger than theirrespective reduction ratio to assure equal final thicknesses of allsamples (processed samples and control samples). The treated sampleswere first annealed at 400° C. for a period of 1 hour. After annealing,these samples were cold rolled to achieve reductions of 0%, 10%, 20%,30%, and 50% of the original thickness of the samples, leaving allsamples with a final thickness of ˜6 mm. Each sample's width and lengthwere adjusted by grinding to be 6 mm by 50 mm. Half of the samples wereheat treated at 500° C. and the other half at 400° C., using an equalnumber of samples from each cold rolling thickness reduction category.After heat treatment, all samples (except the control sample that wasnot sensitized) were sensitized for 100 hours at 150° C.

To prepare all of the samples for the mass loss test, a desmuttingprocess was carried out to remove residual substances on the samples'surfaces. Samples were placed in a sodium hydroxide solution and then innitric acid for sufficient time to clean the surfaces. The samples weresent to be tested for 24 hour mass loss in temperature controlled nitricacid at Carderock's Naval Surface Warfare Center. All the mass loss testprocedures were completed in compliance with the ASTM G67 standard.

The results of the ASTM G67 mass loss test for the treated and controlAA5083 samples show that the sample with the least mass loss were thosethat had been cold rolled to a 10% thickness reduction and heat treatedat 500° C. (FIG. 2). The samples heat treated at 500° C. showed thelowest corrosion loss at lower cold rolling thickness reductionpercentages but corrosion loss increased at a 20% cold rolled thicknessreduction and above. At cold rolling thickness reductions greater than20%, the samples that were heat treated at 500° C. tended to lose moremass in most cases than the samples that were heat treated at 400° C.This trend was confirmed by the data of FIG. 3.

Example 2

In this example, the same sample treatment procedure as Example 1 wascarried out, except that the cold rolling thickness reductionpercentages were 5% and 15%. The results were consistent with theobservations from Example 1 as shown in FIG. 3. The samples cold rolledto a 5% thickness reduction and heat treated at 500° C. outperformed allother samples showing a mass loss per area of less than 10 mg/cm².

EBSD scan images of the samples captured the grain structures within thebest and worst performing samples in terms of mass loss in Example 2.The grain structure of the sensitized control sample (FIG. 4A) showedsmall grain sizes as compared to the relatively large grains of thesample treated with a 5% cold rolling thickness reduction and a 500° C.heat treatment (FIG. 4B).

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meanings of the terms inwhich the appended claims are expressed.

What is claimed is:
 1. A process for grain boundary engineering of analuminum alloy of AA5XXX series, said process comprising steps of:annealing said aluminum alloy at a temperature of from about 350° C. toabout 450° C.; deforming said annealed aluminum alloy to reduce thethickness by from about 2% to about 20% of the original thickness of thealuminum alloy; and heat treating said deformed aluminum alloy at atemperature of from about 450° C. to about 550° C.
 2. The process ofclaim 1, wherein said temperature in said annealing step is from about370° C. to about 430° C.
 3. The process of claim 1, wherein saidannealing step is performed for a period from about 0.5 to about 3hours.
 4. The process of claim 1, further comprising the step of coolingthe annealed aluminum alloy before the deforming step.
 5. The process ofclaim 4, wherein said cooling step has a cooling rate of from about 10to about 30° C. per minute.
 6. The process of claim 1, wherein saiddeforming step comprises cold rolling.
 7. The process of claim 6,wherein said cold rolling reduces the thickness from 3% to 15%.
 8. Theprocess of claim 6, wherein said cold rolling reduces the thickness from5% to 10%.
 9. The process of claim 6, wherein said cold rolling isperformed at room temperature.
 10. The process of claim 1, wherein saidtemperature in said heat treating step is from 475° C. to 525° C. 11.The process of claim 1, wherein said temperature in said heat treatingstep is from 485° C. to 515° C.
 12. The process of claim 1, wherein saidheat treating step is performed for a period of from about 1 to about 6hours.
 13. The process of claim 1, further comprising the step ofsensitizing said heat treated aluminum alloy.
 14. The process of claim13, wherein said sensitizing step is performed at a temperature of fromabout 100° C. to about 200° C.
 15. The process of claim 13, wherein saidsensitizing step is performed for a period of from about 70 to about 150hours.
 16. The process of claim 13, wherein said sensitizing stepcomprising a plurality of sensitizing steps.
 17. The process of claim16, wherein each of said plurality of sensitizing steps is performed ata different temperature.
 18. The process of claim 16, wherein each ofsaid plurality of sensitizing steps is performed with a low temperaturesensitizing step followed by sensitizing steps at higher temperatures.19. The process of claim 16, wherein each of said plurality ofsensitizing steps is performed using a cooling step following eachsensitizing step.
 20. An aluminum alloy of AA5XXX series treated by theprocess of claim 1.